Trophic Ecology of Four Apex Predators of the Pelagic Ecosystem in the Indian Ocean

Transcription

1 Trophic Ecology of Four Apex Predators of the Pelagic Ecosystem in the Indian Ocean Lancet fish, Yellowfin tuna, Bigeye tuna & Swordfish Michel Potier 1, Francis Marsac 2, Vincent Lucas 3, Richard Sabatié 4, Olivier Maury 1, Jean-Pierre Hallier 1, Yves Chérel 5 & Frédéric Ménard 1 1 IRD, Reunion Island 2 IRD, Sète, France 3 Seychelles Fishing Authority, Seychelles 4 ENSA, Rennes, France 5 CNRS-CEBC, Chizé, France

2 «Who eats who?» is a relevant approach to understand the characteristics of an ecosystem and how the different functional groups interact No food, No future.

3 Yellowfin (Thunnus albacares) Lancetfish (Alepisaurus ferox) Goals Compare the diet of four major predators Study the degree of overlap- ping of feeding regimes among predators Estimate the size ratios between predators & prey Introduce a stable isotope perspective Swordfish (Xiphias gladius) Bigeye (Thunnus obesus)

4 Vertical Distribution : where do they live? What we know from ultrasonic tagging % The 4 predators occupy different depth ranges : Depth (m) Yellowfin : m Bigeye tuna : m Swordfish : m Lancet fish :??

5 10 5 Status May (20) Traine 70 Palangre (497) Senne (518) Study area Number of stomachs collected > 1100

6 Survey implementation months (Aug. 01/Feb. 03) 67 LL sets and 10 PS sets 1 oceanographic cruise R/V 47 CTD profiles (T & DO) 15 midwater trawls -6-8 Trawls No of stomachs < 5 PS sets collected by 5-15 longline set > JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Total Lancet fish Yellowfin Bigeye Swordfish Total non-empty stomachs LL (nb) % empty Size range (cm) non-empty stomachs PS (nb) % empty 0 0 Size range (cm)

7 The environmental context : Temperature & DO profiles and depth range sampled by the longline gear Temperature Dissolved oxygen

8 Methods 1- Dietary Analysis A B Weighing the stomach content Sorting in large categories Fish Molluscs Crustacea The less abundant categories (ie. molluscs and crustacea) are weighed then the main category (ie. fish) is obtained by difference C Accumulated food For each category sorting Fresh Food Counting the remarkable organs (otoliths, beaks, mandibles,, ) For each item Species identification, counting, measuring the remarkable organs or entire individuals. Reconstituted weight of the diet

9 Methods 2- Prey Dominance Indices Modified Costello diagram (Amundsen et al. 1996) IRI Index of Relative Importance (IRI) (Pinkas et al. 1971) A ( N A% PA %) FA% Number N, Weight P, Occurrence F

10 IRI A ( A A A Methods N % P %) F % 3- Similarity Index Morisita & Horn Index of similarity (Magurran 1988) C mh ( da 2 S i1 an i bn db)( an i bn) S : total no of prey in regime of both predators an : total no of prey in diet of predator A bn : total no of prey in diet of predator B an i : no of individuals prey i in diet predator A bn i : no of individuals prey i in diet predator B da, db : quadratic ratios Feeding regimes are compared by pairs of predators Varies from 0 (distinct feeding regimes) to 1 (identical) Above 0.6, C is reflecting a significant overlap

11 Methods 4- Size Measurements Fresh Items Morphometric relationships Remainings Crustaceans Propod (pelagic crabs) Total length Propod (pelagic crabs) Telson length Fish Standard length Otolith length Dentary length Cephalopods Lower rostral length (LRL) Lower rostral length (LRL)

12 Methods 5- Stable isotope ratios 15 N ( ) Intermediate trophic levels Prey 2 Prey Zoo? 1 Phyto? POM 1? Tuna samples 3-4? C ( ) Samples of main prey, tuna and billfishes (white muscle), sharks POM and Phytoplankton collected but not yet analysed Sampling of filter feeders is underway

13 Results 1 Prey composition and dominance studied with the IRI and Costello diagram YFT and BET across gears 4 predators with longline only

14 Comparison 2 pred. (YFT/BET) across gears : PS blank bars/ll colored bars %IRI % IRI Natosquilla investigatoris crab larvae crustacea Natosquilla investigatoris Oplophorus typus Charybdis edwardsi other Myctophids Hyperiids Diretmoides parini Odontodactylus scyllarus Cubiceps pauciradiatus Scombrids unknown fish Cubiceps pauciradiatus Paralepis atlanticus Alepisaurus ferox Ommastrephids Yellowfin LL = 44 p.i. PS = 3 p.i. fish cephalop. Scopelarchus analis Paralepis atlanticus unknown squids Ommastrephids octopods Onychoteuthids Bigeye LL = 21 p.i. PS = 2 p.i. 100 %IRI 80 Yellowfin Bigeye longline purse seine longline purse seine

15 Surface dwelling tuna (open circles) Dominance of Natosquilla Specialisation on Scombrids (YFT) and flying squids (BET) Deep dwelling tuna (black circles) YFT : from generalized to specialisation (swim.crabs and crab larvae) BET : generalized feeding behaviour

16 Comparison between 4 predators, longline gear 100% % IRI 80% 60% 40% others squids fish crustacea 20% 0% lancet fish yellowfin tuna bigeye tuna swordfish predators sp. The proportion of each prey group (in wet mass) among the predators is consistent with the pattern depicted by the IRI

17 IRI by prey item and predator Charybdis edwardsi Oplophorus typus Natosquilla investigatoris Natosquilla investigatoris Platyscelus ovoides Scopelarchus analis Phrosina sp. Alepisaurus ferox Omosudis lowei Paralepis atlantica Fish larvae Carinaria sp. Alciopidae Lancet fish 1868 prey 41 families crustacea fish Paralepis atlantica Diaphus spp. Cubiceps pauciradiatus Diretmoides parini Onykia rancureli Moroteuthis lonnberggii Sthenoteuthis oualaniensis bigeye tuna 154 prey 18 families Crab larvae Charybdis edwardsi Phrosina sp. Odontodactylus scyllarus Other scombrids Cubiceps pauciradiatus Diaphus spp. Alepisaurus ferox Sthenoteuthis oualaniensis Ornithoteuthis volatilis Acanthephyra sp. Natosquilla investigatoris Cubiceps pauciradiatus cephalop. Diretmoides parini Und. Fish Scopelarchus analis 4916 prey Sthenoteuthis oualaniensis 44 families Ornithoteuthis volatilis 818 prey Yellowfin tuna Moroteuthis lonnbergii Ancistrocheirus lesueuri swordfish 28 families

18 % Natosquilla investigatoris (Stomatopod) Only documented in the Indian Ocean Demographic bursts in 1906, 1933, 1944,1967 A new episode underway since 1999 May cm 3 cm 4 cm 7 cm October Swarming at the surface Total length Telson size mm Charybdis edwardsi (Portunidae) Feb-Mar larvae adults Jul-Jan J F M A M J J A S O N D Swarming in the mixed layer

19 Results 1 Prey composition and dominance studied with the IRI and Costello diagram 2 - Overlap of feeding regimes

20 Similarity indices and cluster analysis YFT LL BET LL YFT PS BET PS YFT LL 1 BET LL YFT PS BET PS Lancet fish Yellowfin Bigeye Swordfish Lancet fish 1 Yellowfin Bigeye Swordfish Significant overlap within the PS Values close to the significant threshold between bigeye and swordfish Cluster analysis based on IRIs Aggregation Distance

21 Fish Prey Distribution Epipelagics Mesopelagics Fish larvae Acanthuridae Balistidae Carangidae Carapidae Coryphaenidae Dactylopteridae Diodontidae Exocoetidae Hemiramphidae Holocentridae Monacanthidae Ostraciidae Scombridae Scorpaenidae Tétraodontidae Alepisauridae Anoplogasteridae Argentinidae Bramidae Chiasmodontidae Diretmidae Gempylidae Myctophidae Nomeidae Omosudidae Paralepididae Phosichthyidae Scopelarchidae Sternopthychidae Stromateidae Trachichthyidae Swordfish Lancet fish Bigeye Yellowfin yellowfin 42% swordfish 10% big eye 0% swordfish 29% big eye 31% Epipelagics lancet fish 19% lancet fish 48% Mesopelagics yellowfin 21%

22 The differences / similarities between the vertical distribution of predators are reflected in the prey composition : yellowfin/lancet fish with a dominant proportion of crustaceans : epipelagic affinities Bigeye/swordfish with mesopelagic fish dominating the diet Fish is the most diversified group among prey : 24 families reported in the YFT diet Ocean Area Fish Families Authors Atlantic Gulf of Guinea 20 Borodulina (1974) Pacific Eastern Pacific 42 Alverson (1963) Eastern Pacific 18 Moteki et al. (2001) Central Pacific 38 Reintjes and king (1953) Central Pacific 48 King and Ikehara (1956) Western Pacific 37 Watanabe (1958) Western Pacific 30 Borodulina (1982) Western Pacific 13 Kim et al. (1997) Indian Western Indian 23 Kornilova (1980) Sri Lanka 44 Maldenya (1996) Seychelles 24 Present study

23 Results 1 Prey composition and dominance studied with the IRI 2 - Overlap of feeding regimes 3 - Size spectrum of prey

24 Prey sizes : trawls vs stomachs 70 % Size spectra of fish preys SL (cm) trawl lancet fish swordfish big eye tuna yellowfin tuna

25 % Size spectra of fish preys SL (cm) trawl lancet fish swordfish big eye tuna yellowfin tuna Swordfish and bigeye ingest bigger prey than do yellowfin and lancetfish. Lancetfish is a rather slow swimmer and prey upon small organisms (apart from its conspecifics). Swordfish has a very efficient method of capture involving the whipping action of the bill. The real size spectrum of prey in the water column is the overall combination of trawl collections and predator stomach contents.

26 Predator-prey size ratios Common size ratios found in FishBase : around 4 (Froese and Pauly 1998) In the present study, size ratios range from 10.3 to 11.8 These high ratios can be interpreted as an optimal feeding strategy : preying upon dense swarms reduces the energetic cost of chasing The densest prey concentrations in the open ocean are made of small-sized individuals

27 Results 1 Prey composition and dominance studied with the IRI 2 - Overlap of feeding regimes 3 Size analysis 4 Stable Isotope ratios

28 10 5 N = 108 (Status Nov 2003)

29 Effect of lipid removal on 13 C measurements white muscle samples l i s t a o / K E l i s t a o / K E l i s t a o / A M 2-5 L I S 1 2 l i s t a o / K E l i s t a o / K E l i s t a o / G U a l b a c o r e / K E l i s t a o a l b a c o r e / A M 4-5 A L B 3 a l b a c o r e / K E a l b a c o r e / A M 1-4 a l b a c o r e / A M 1-1 a l b a c o r e / G U a l b a c o r e / G U a l b a c o r e / G U D p a t u d o a l b a c o r e / A M 3-2 a l b a c o r e / A M 3-1 e s p a d o n / A M 2-7 E S P 9 e s p a d o n / A M 2-3 E S P 1 1 e s p a d o n / A M 2-1 E S P 7 e s p a d o n / A M e s p a d o n / A M % d e l t a. P D B a l b a c o r e e s p a d o n l i p i d s %

30 ð15n ( ) TL Top predators Prey + lancetfish 1.5 Zoo ð13c ( ) YFT BET ALE Lancet fish Cubiceps Vinciguerria squilles Mantis shrimp Zooplankton CAF Silky shark Enoploteuthidae MAN Black marlin VOI Sailfish TL consumer = 1 + (D consumer D POM )/TL D POM = 5.4 (litterature) TL = 3.52 (estimated)

31 Summary Surface tunas exploit a very limited number of prey species : strong influence of the mantis shrimp episode Two functional groups utilizing different prey communities : adult yellowfin & lancetfish on one side, adult bigeye & swordfish on the other side The optimal feeding strategy, the high pred-prey size ratios could reveal short and simple food chains leading to apex predators in the IO equatorial ecosystem (Phytopk Zoopk Crustacean Yellowfin). Needs confirmation from the isotopes.

32 Next steps Implementation of a long-term monitoring of key descriptors of the trophic pathways : tracking the shifts in maximizing the benefit/cost ratio ; Determine the baseline isotope signature of the ecosystem ; Integration of observations in a spatially explicit and size structured ecosystem model.

33 The APECOSM trophodynamic components Spatially explicit pelagic ecosystem (PE) Spatially explicit PE weight spectrum dynamics (by funct. Groups). OOPE biomass (log) 15 N Spatially explicit PE 15 N weight spectrum dynamics. Organisms weight (log) Predators sample the PE: their stomach content weight spectra (by main prey functional groups) and 15 N dynamics are spatially explicit. The model parameters will be estimated using all the stomach content and isotope data collected